The present invention relates to a magnetic recording medium and a magnetic storage apparatus, specifically to a magnetic recording medium having a recording density of 50 Gbits or more per square inch and to a magnetic storage apparatus incorporating the same.
In recent years, an areal recording density of a magnetic disk device as an external storage apparatus of a computer is increased by 100% per year. However, as the areal recording density is increased, a problem that data magnetically recorded is erased by circumferential heat, that is, a so-called thermal fluctuation, has become obvious. Accordingly, the conventional longitudinal recording method has been considered to be difficult to achieve the areal recording density exceeding 50 Gbits per square inch.
On the other hand, unlike the longitudinal recording method, a perpendicular recording method has a feature that, as a linear recording density is increased, a demagnetizing field acting between adjacent bits is decreased to stabilize recorded magnetization. Accordingly, the perpendicular recording method is considered to be one of the effective means for exceeding the thermal fluctuation limit of the longitudinal recording method.
In the perpendicular recording method, a combination of a single pole type head and a double-layer perpendicular medium composed of a soft magnetic underlayer and a perpendicular recording layer is effective in realizing high density recording. However, since the double-layer perpendicular medium includes the soft magnetic underlayer of a high saturation magnetic flux density (Bs), following problems have been pointed out: a stray field generated from domain walls of the soft magnetic underlayer is observed as spike noises, or recorded magnetization disappears by displacement of domain walls of the soft magnetic underlayer. As a method for solving these problems, for example as disclosed in Japanese Patent Laid-Open Nos. 07(1995)-129946 and 11(1999)-191217, it has been proposed that a hard magnetic pinning layer is provided between the soft magnetic layer and a substrate so that magnetization directions of the soft magnetic layer are aligned with one direction. As disclosed in Japanese Patent Laid-Open No. 06(1994)-103553, a method has been proposed, in which the displacement of domain walls of the soft magnetic layer is suppressed by an exchange coupling with an antiferromagnetic layer having magnetic spins aligned with each other.
However, in the method of aligning magnetization directions of the soft magnetic underlayer by use of the hard magnetic pinning layer, magnetic domains having an opposite magnetization direction are likely to be formed around inner and outer edges a disk substrate, and spike noises therefrom are observed. On the other hand, the method of suppressing the displacement of domain walls of the soft magnetic underlayer by use of the antiferromagnetic layer has an effect for preventing the disappearance of the recorded magnetization, which is caused by the displacement of domain walls, but cannot prevent the spike noises attributable to the domain walls.
The present invention is made to solve the above problems. Specifically, an object of the present invention is to provide a perpendicular magnetic recording medium having a recording density of 50 Gbits or more per square inch and a high medium SIN, which suppresses spike noises from the soft magnetic underlayer by a magnetic domain control layer and to provide a manufacturing process of the same, so as to facilitate realization of a high-density magnetic storage apparatus.
In the perpendicular magnetic recording medium including a domain control layer, an amorphous soft magnetic underlayer, and a perpendicular recording layer, which are sequentially formed on a substrate, the domain control layer is a triple-layer film including a first polycrystalline soft magnetic layer, a disordered antiferromagnetic layer, and a second polycrystalline soft magnetic layer, which are sequentially formed from a substrate side, so that domain control of the soft magnetic underlayer and reduction of medium noises can be achieved.
The inventors found out that the domain control layer as the above triple-layer film was effective as a result of investigation of various kinds of method for the domain control of the amorphous soft magnetic underlayer. Each of the first and the second polycrystalline soft magnetic layers is required to be capable of offering a soft magnetic property at a small film thickness and to have a good lattice matching with the disordered antiferromagnetic layer. Specifically, for the first and the second polycrystalline soft magnetic layers, a face-centered cubic (fcc) alloy mainly composed of Ni and Fe or an fcc alloy mainly composed of Co can be employed. Examples of these alloys include a Ni81Fe19 alloy, a Ni80Fe20 alloy, Co, and a Co90Fe10 alloy. Here, a numeral following a symbol of an element means a content of the element in atomic percent.
At formation of the disordered antiferromagnetic layer, an interlayer exchange coupling is necessary to act between the disordered antiferromagnetic layer and the first polycrystalline soft magnetic layer. Specifically, for the disordered antiferromagnetic layer, a disordered alloy mainly composed of Mn and Ir, or a disordered alloy mainly composed of Cr, Mn, and Pt can be employed. When the domain control layer is formed using such a material while applying a magnetic field having a component of a parallel direction to a surface of the substrate, a unidirectional magnetic anisotropy is induced in a direction of applying the magnetic field, so that the magnetization directions of the first and the second polycrystalline soft magnetic layer can be aligned with the direction of the applied magnetic field. Specifically, when the domain control layer using the above material is formed by a magnetron sputtering method, the magnetization directions of the first and the second polycrystalline soft magnetic layers can be aligned with a direction of a stray field from a cathode, that is, a radial direction of the disk substrate. As described above, the spike noises can be effectively suppressed by providing the domain control layer with a unidirectional magnetic anisotropy. On the other hand, since the ordered antiferromagnetic alloy such as a PtMn alloy and a NiMn alloy is generally in a disordered state at film formation, the exchange coupling does not act between the antiferromagnetic layer using such materials and the first polycrystalline soft magnetic layer. Accordingly, after the film formation, an ordering heat treatment is required for several hours while applying a magnetic field. Such a step is not desirable because a medium manufacturing process is made complicated and then costs are increased.
In the case of using the magnetron sputtering apparatus, the amorphous soft magnetic underlayer is provided with a uniaxial magnetic anisotropy having an easy axis of magnetization along the radial direction of the disk substrate during the medium manufacturing process. In the case where the domain control layer is not provided, several spoke-like 180xc2x0 domain walls exist on the disk substrate so as to lower magnetostatic energy. By using the domain control layer of the present invention, the exchange coupling acts between the second polycrystalline soft magnetic layer and the amorphous soft magnetic underlayer, so that the amorphous soft magnetic underlayer is provided with the unidirectional magnetic anisotropy having the easy direction of magnetization aligned with the magnetization direction of the second polycrystalline soft magnetic layer. Accordingly, the spoke-like domain walls can be removed except the inner and outer edges of the disk substrate. Since magnetic poles are formed at the edges of the disk substrate, magnetic domains are formed so as to lower magnetostatic energy. However, in the medium of the present invention, an area having the magnetic domains formed thereon can be suppressed within areas of 1 mm from the edges of the disk substrate, that is, areas other than a data area. As a film thickness of the amorphous soft magnetic underlayer is increased, the exchange coupling force from the domain control layer is relatively decreased, so that the domain formation area around the edges of the disk substrate is expanded. In such a case, the amorphous soft magnetic underlayer is divided into two layers, and the domain control layer is inserted therebetween, thus reducing the domain formation area around the edges of the disk substrate. A material of the amorphous soft magnetic underlayer is not particularly limited, as long as the material is an amorphous alloy having Bs at least not less than 1 tesla (T), having an excellent surface flatness, and not crystallizing in the medium manufacturing process. Specifically, an amorphous alloy, which is mainly composed of Fe or Co and to which Ta, Hf, Nb, Zr, Si, B, or the like are added, can be employed.
For the intermediate layer used in the perpendicular recording medium of the present invention, a nonmagnetic alloy, which is amorphous or has a hexagonal closed packed (hcp) structure, can be employed. For the perpendicular recording layer, a CoCrPt alloy, a Co/Pd multilayer film, a Co/Pt multilayer film, or the like can be employed. In particular, since each of the Co/Pd multilayer film and the Co/Pt multilayer film of a small thickness offers a high coercivity not less than 5 kOe, resolution can be improved.
As for a protective layer of the perpendicular recording layer, a film mainly composed of carbon is formed with a film thickness ranging from 3 nm to 10 nm, and a lubricating layer is further formed of perfluoroalkylpolyether or the like at a film thickness ranging from 1 nm to 10 nm, whereby the perpendicular magnetic recording medium, which has an excellent reliability, can be obtained.
A magnetic storage apparatus of the present invention includes the above-described perpendicular magnetic recording medium; a driving unit for driving the perpendicular magnetic recording medium in a recording direction; a magnetic head composed of a recording section and a reproducing section; a unit for relatively moving the magnetic head with respect to the perpendicular recording medium; and a recording and reproducing processing unit for inputting signals to the magnetic head and for reproducing output signals from the magnetic head. The reproducing section of the magnetic head is composed of a high sensitivity element utilizing a magnetoresistive effect or a tunneling magnetoresistive effect. Accordingly, the magnetic storage apparatus can be realized, which has an excellent reliability at the recording density of 50 Gbits or more per square inch.